1. Field of the Invention
The present invention relates to an exhaust-gas purification device for an internal combustion engine or the like. More particularly, the present invention relates to a device which can effectively purify the hydrocarbons (HC) among the toxic components in the exhaust gas emitted by an internal combustion engine or the like and can prevent said hydrocarbons from being released into the air even when the temperature of said exhaust-gas is relatively low (for example, immediately after the start of the engine).
2. Description of the Related Art
For purifying the exhaust-gas of automobiles, catalysts which carry noble metals (platinum, rhodium etc.) or the other metals are conventionally used. Such catalysts purify the toxic components of the exhaust-gas by performing the oxidation or reduction of these components (HC, CO, NO.sub.x). Particularly, the catalyst purification of hydrocarbon (HC) is greatly influenced by exhaust-gas temperatures, and environmental temperatures from 350.degree. to 400.degree. C. and over are necessary for the purification of hydrocarbon (HC). Accordingly, immediately after the start of an internal combustion engine, the purification of hydrocarbon (HC) is not performed since the exhaust-gas temperature is so low that the catalyst used cannot reach the temperature above its activation temperature (350.degree.-400.degree. C.). Moreover, since the emission quantity of hydrocarbon (HC) is very large in the low-temperature condition of the engine immediately after its start, there arises a problem that a great quantity of hydrocarbon (HC) may be released into the air while the exhaust-gas temperature is low in the above-mentioned low-temperature period. Consequently, in order to solve the above-mentioned problem, known exhaust-gas purification devices have been proposed which comprise conventional catalysts placed in engine exhaust systems and HC trappers placed upstream from said catalysts. These HC trappers contain absorbents for absorbing the hydrocarbon (HC) released in the low-temperature condition of engines. (Cf. e.g. Unexamined Patent Publication No. 2-75327, No. 3-194113 and the like).
In the exhaust-gas purification device disclosed in the Unexamined Patent Publication (Kokai) No. 2-75327, a zeolite absorbent is placed upstream from a catalyst and this absorbent is used in combination with this catalyst. In this device, as long as exhaust-gas temperature is low, said absorbent traps hydrocarbon (HC), and as long as the exhaust-gas temperature is high, the catalyst purifies both the hydrocarbon (HC) which is desorbed from the absorbent and the hydrocarbon (HC) which is emitted by the engine. On the other hand, in the exhaust-gas purification device disclosed in the Unexamined Patent Publication (Kokai) No. 3-194113, an absorbent and a catalyst are placed in parallel. An exhaust-pipe containing the absorbent is connected to an exhaust-pipe containing the catalyst so that the exhaust-pipe containing the absorbent bypasses from the upstream section to the downstream section of the exhaust-pipe containing the catalyst. Furthermore, a selector valve is placed in the connecting section (i.e., the downstream junction) of the above-mentioned two exhaust pipes. During a certain period immediately after the start of the engine, the opening of the selector valve is adjusted so that the absorbent can absorb hydrocarbon (HC). When the exhaust-gas becomes hot, the selector valve is half-opened and this hot exhaust-gas flows into the absorbent. Thereby the hydrocarbon (HC) absorbed in the absorbent can be desorbed from the absorbent. Then the hydrocarbon (HC) desorbed is returned to the area upstream from the catalyst and is purified.
Since the above-mentioned known devices for absorbing and desorbing the hydrocarbon (HC) emitted from an engine or the like in its low-temperature condition (hereinafter referred to as "cold HC") is placed upstream from the catalyst, these devices must be heat-resistant. Therefore, in the devices disclosed in the Unexamined Patent Publication (Kokai) No. 2-75327 and the like, zeolite absorbents are used which are highly heat-resistant. However, in an extremely low temperature condition (especially less than 50.degree. C.), the hydrocarbon (HC) absorption performance of zeolite is not always suitable, and activated carbon provides better results than zeolite. Furthermore, even in the case of using other kinds of absorbents, it is disadvantageous to the improvement of the hydrocarbon (HC) absorption ability to place such absorbents upstream from the catalyst since the absorption performance of those absorbents becomes better as their temperatures fall.
Furthermore, in the devices disclosed in the above-mentioned patent publications, because the desorption of the absorbed substances depends on the heat of the exhaust gas, the desorption speed must be low and it takes time to restore the absorbents.